Method and device for controlling an internal combustion engine with variable valve lift and motor vehicle equipped therewith

ABSTRACT

A method for controlling an internal combustion engine with variable valve lift has the following steps: Determining the pressure in the intake manifold; predetermining the pressure in the intake manifold following a changeover of the valve lift; determining a selection criterion for a valve lift to be set; and setting that valve lift at which the higher pressure is reached in the intake manifold, unless the selection criterion requires the use of the valve lift at which the lower pressure is reached in the intake manifold.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to German Patent Application No. 102008 003 832.6 filed Jan. 10, 2008, the contents of which isincorporated herein by reference in its entirety.

TECHNICAL FIELD

The invention relates to a method and a device for controlling aninternal combustion engine with variable valve lift. Such engines withtwo- or multi-stage valve lift changeover can be operated almostdethrottled in a greater number of operating states compared withinternal combustion engines with fixed valve lift. This mode ofoperation has a good degree of efficiency, as is known. The invention isconcerned in particular with the task of determining the operatingpoints at which changeover from the one to the other valve lift is totake place.

BACKGROUND

A method is known from the prior art for adapting an internal combustionengine to different operating states by providing a two- or multi-stagevalve lift changeover. For example, an electrohydraulic valve liftchangeover can be implemented by actuating a locking element into ashift cam follower by means of oil pressure against a spring. Accordingto the activation or deactivation state, the cam follower switchesbetween two different elevation cams of the camshaft. To effect thechangeover, a magnetic valve situated in the oil circulation system isenergized, which opens thereupon. The oil pressure builds up and thelocking element moves against a spring until the locking process hasbeen completed. When the magnetic valve is closed again, the oilpressure decreases by way of an oil leakage pipe and the locking elementslides back into its home position as the result of spring force. Apartfrom this described two-stage system, multi-stage or continuouslyoperating valve lift changeovers are also commonly encountered.

A further method is known from the prior art for storing the operatingpoints for the switch from one nominal value for the valve lift toanother nominal value in characteristic maps and switching in accordancewith the operating state of the engine. According to the prior art,possible parameters for controlling the valve lift changeover are theair mass nominal value and the engine rotational speed. For example, inthe case of a low air mass nominal value a small valve lift is selectedand in the case of a high air mass nominal value a large valve lift isselected. Between these, a changeover which is as torque-neutral aspossible is aimed for.

SUMMARY

According to various embodiments, an optimized method and also a devicefor implementing the method can be set down, with which the operatingpoints for the switch in the valve lift can be determined more reliably.Furthermore, according to various embodiments, a method and also adevice for controlling an internal combustion engine with variable valvelift can be set down, through which pollutant emissions can be reducedand fuel can be saved.

According to an embodiment, a method for controlling an internalcombustion engine with variable valve lift, may comprise the followingsteps: Determining the pressure in the intake manifold for the currentvalve lift; Predetermining the pressure in the intake manifold followinga changeover of the valve lift; Determining a selection criterion for avalve lift to be set; and Setting the valve lift at which the higherpressure is reached in the intake manifold, unless the selectioncriterion requires the use of the valve lift at which the lower pressureis reached in the intake manifold.

According to a further embodiment, the valve lift can be selected atwhich the lower pressure is reached in the intake manifold if thecurrent torque delivery in the valve lift at which the higher pressureis reached in the intake manifold cannot be achieved. According to afurther embodiment, the valve lift can be chosen at which the lowerpressure is reached in the intake manifold if a registered driver inputcannot be implemented in the valve lift at which the higher pressure isreached in the intake manifold. According to a further embodiment, thevalve lift can be chosen at which the lower pressure is reached in theintake manifold in order to prevent the internal combustion engine fromentering an operating state which cannot be set with the two possiblesettings for the valve lift. According to a further embodiment, theinternal combustion engine can be operated with the valve lift at whichthe higher pressure is reached in the intake manifold if the internalcombustion engine is operated at an operating point having a rotationalspeed below the intersection point of two mass flow curves. According toa further embodiment, at the operating point having a rotational speedbelow the intersection point of two mass flow curves approx. 5% toapprox. 40%, in particular approx. 10% to approx. 30%, of the maximumtorque can be delivered. According to a further embodiment, the currentpressure in the intake manifold and the pressure resulting after a valvelift changeover can be determined at predefinable time intervals.According to a further embodiment, the time interval may be approx. 5 msto approx. 100 ms. According to a further embodiment, the currentpressure in the intake manifold and the pressure resulting after a valvelift changeover can be determined in an event driven manner. Accordingto a further embodiment, the event can be selected from a driver inputand/or a change in the drive position and/or the switching state of aconsumer load and/or the intervention of a slip control facility.

According to another embodiment, a device for controlling an internalcombustion engine with variable valve lift, may comprise a facility forregistering the torque and also the pressure in the intake manifold, andalso means for predetermining the pressure following a changeover of thevalve lift, whereby means are provided for setting the valve lift forthe purpose of setting that valve lift at which the higher pressure isreached in the intake manifold, unless means for predetermining thevalve lift nominal value require the use of the valve lift at which thelower pressure is reached in the intake manifold.

According to a further embodiment, the means for predetermining thevalve lift nominal value can be set up in order to check whether thecurrent torque delivery can be achieved with the valve lift at which thehigher pressure is reached in the intake manifold. According to afurther embodiment, means can be provided for registering a driver inputand the means for predetermining the valve lift nominal value are set upin order to check whether a registered driver input can be realized withthe valve lift at which the higher pressure is reached in the intakemanifold. According to a further embodiment, the means forpredetermining the valve lift nominal value can be set up in order tocheck whether torque-neutral changeover of the valve lift is possible.According to a further embodiment, the means for predetermining thevalve lift nominal value and/or the means for predetermining thepressure following a changeover of the valve lift may comprise a numericcharacteristic map matrix and/or a microprocessor and/or a neuralnetwork. According to a further embodiment, a timer can be provided fordetermining a predefinable period of time.

According to yet another embodiment, a motor vehicle may have such adevice for controlling an internal combustion engine as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in detail in the following withreference to an embodiment.

FIGS. 1 and 2 show the relationship between the air mass taken in andthe engine rotational speed for two different settings of the valvelift.

FIG. 1 shows entered in bold print the particular valve lift whichresults with regard to the valve lift control in accordance with the airmass taken in.

FIG. 2 shows emphasized in bold print the particular setting for thevalve lift which is selected in accordance with the method.

DETAILED DESCRIPTION

According to various embodiments, in a method for controlling aninternal combustion engine with variable valve lift, the pressureprevailing in the intake manifold is initially determined. Subsequently,in the engine control unit the pressure in the intake manifold isdetermined following a changeover in the valve lift. Subsequently, thatvalve lift is always set at which the higher pressure is reached in theintake manifold, unless a selection criterion exceptionally exists whichrequires the use of that valve lift at which the lower pressure isreached in the intake manifold.

Furthermore, according to other various embodiments, a device forcontrolling an internal combustion engine with variable valve lift, hasa facility for registering the torque and also the pressure in theintake manifold, and also means for predetermining the pressurefollowing a changeover of the valve lift, whereby means are provided forsetting the valve lift for the purpose of setting that valve lift atwhich the higher pressure is reached in the intake manifold, unlessmeans for predetermining the valve lift nominal value require the use ofthe valve lift at which the lower pressure is reached.

According to various embodiments, it has been recognized that optimizedoperation of an internal combustion engine is achieved with a pluralityof possible switching states for the valve lift in a particularly simplemanner if, instead of the air mass nominal value, the pressure in theintake manifold is employed for taking the decision as to whether or nota valve lift changeover is required. In this situation, it has becomeapparent that the decision in favor of a valve lift changeover as aresult of the intake manifold pressure can manage with the simplecriterion of choosing as a rule that valve lift at which the higherintake manifold pressure is reached. Only in a few, selected operatingstates which need to be monitored separately in the engine control unitshould a different valve lift exceptionally be chosen. As a rule, theoperating state with the highest intake manifold pressure is thatoperating state in which the throttle valve exhibits the greatestopening. The operating state with the highest intake manifold pressureis thus characterized by the fact that the smallest throttling lossesoccur.

Although the invention is described by way of example of a two-stagevalve lift changeover, the person skilled in the art will also be ableto apply the disclosed principle without further ado to multi-stage orvariable valve lift changeovers.

The necessary determination of the current pressure in the intakemanifold can be effected according to various embodiments in anespecially simple manner by means of a pressure sensor in the intaketract. Depending on the measuring location, it may become necessary inthis situation to correct the measured pressure in the engine controlunit to a pressure actually present at the inlet valve. To this end, themeasured value can be calculated with correction values or a correctedvalue from the measured value can be read out from a characteristic map.In an alternative embodiment, the determination of the current pressureis however also indirectly possible through a characteristic map or aneural network or through calculation from other measurement variables.For example, the current pressure in the intake manifold can beascertained depending on the rotational speed, the air mass and thetemperature in road trials and/or testbed trials and then ascertainedfrom these measurement variables by the control unit during operation ofthe internal combustion engine.

In order to decide whether the pressure in the intake manifold is higherat the currently selected setting for the valve lift or at a differentsetting for the valve lift, the intake manifold pressure must beascertained at the other valve lift setting in each case. By preference,but not mandatorily, this ascertainment takes place either throughcomputation, through reading out from a characteristic map or by meansof a neural network. Alternatively, the engine control unit can alsoinitiate the changeover to the other valve lift in each case on a trialbasis and determine the intake manifold pressure which is reached as aresult by means of measurement. Provided the intake manifold pressure inquestion is higher in this situation, no changeover back to the valvelift originally chosen takes place. In the case of a lower intakemanifold pressure, a changeover back is initiated after several enginecycles.

According to an embodiment, the determination of the pressure in theintake manifold for two valve lifts can take place not only for thecurrent operating point of the internal combustion engine but also foran operating point present in the future if the latter is reached in theforeseeable future or could be reached with a certain probability, forexample as a result of an already registered driver input.

In order to avoid the changeover to a valve lift which is inappropriatein the individual case, a selection criterion continues to be determinedwhich avoids such a changeover in selected operating states, even if anin itself advantageous increase in the intake manifold pressure wereassociated with this. Such operating points can for example be stored inthe engine control unit. For example, such an operating state is presentif the currently required torque delivery in the case of the valve liftat which the higher pressure is reached in the intake manifold cannot beachieved. In this case, a changeover must not happen in order not toprovoke a collapse of the torque.

Furthermore, a changeover must also not happen in the situation when aregistered driver input leads to the expectation that an implementationof the torque delivery possibly required in the future cannot berealized with the valve lift at which the higher pressure is reached inthe intake manifold. For this purpose the engine control unit preferablycontains forecasting means in order to estimate with the highestpossible degree of accuracy a torque delivery becoming necessary in thefuture.

According to an embodiment, operating states are also avoided from whicha changeover is no longer possible without changing the deliveredtorque. For example, this can be an operating point in the upperpartial-load operational range, which can still be represented with alow valve lift but with which, on account of the great air massdifference from an operating point having the same rotational speed andtorque with a high valve lift, a torque-neutral changeover is no longerpossible. Taking such a blind alley is reliably prevented by thepreferred development.

Furthermore, provision can be made to omit an island point, at which thepreferred increase in pressure is reached only in a narrow rotationalspeed range, from a changeover and thus to minimize the number ofchangeover processes.

It has been found, completely surprisingly, that contrary to the controlmethods used previously the fuel consumption and the pollutant emissionscan also be reduced in the lower partial-load operational range, atcomparatively low torques, if the internal combustion engine is operatedwith a large valve lift and dethrottling. As a result of the small airmass taken in at a low rotational speed, a higher intake manifoldpressure is then reached again than is the case with the same operatingpoint with a small valve lift. In this situation, an operating pointwith low torque delivery is understood to be an operating point at whichapprox. 5% to approx. 40%, in particular approx. 10% to approx. 30%, ofthe maximum torque is delivered.

The decision as to whether or not a valve lift changeover is requiredcan be taken by the engine control unit, at predefinable time intervalsfor example. In particular, such a time interval can be approx. 5 ms toapprox. 100 ms. In this situation, either a fixed time interval can bechosen or the time interval can be adapted dynamically to the operatingconditions, the rotational speed for example. Such a rotational speeddependent time interval can for example be realized by the fact that theintake manifold pressure is determined every n crank rotations. Whenchoosing the number n, the person skilled in the art will in particularconsider a value from the range from approx. 2 to approx. 100.

In an alternative embodiment, the current pressure in the intakemanifold and the pressure in the intake manifold arising in a valve liftchangeover can also be determined in an event driven manner. The eventupon which the previously set valve lift is subjected to checking canfor example be a change in the driver input and/or a change in the driveposition and/or a switching state of a consumer load and/or thedetection of wheel slip. This listing is naturally not exclusive.Rather, any event having an influence on the torque delivery or thetorque nominal value for the internal combustion engine comes intoconsideration. The proposed method according to various embodiments forcontrolling an internal combustion engine is particularly suitable forinternal combustion engines which are incorporated in a motor vehicle.In addition, use in boats or small aircraft is also possible. Thevarious embodiments are particularly suitable for use in conjunctionwith an internal combustion engine having spark ignition.

FIG. 1 shows the air mass taken in by the internal combustion engine onthe vertical axis plotted against the rotational speed of the internalcombustion engine. The maximum possible intake air mass changes as aresult of the valve lift set. In this situation, the first, gentlerlinear slope, which is designated as “LOW”, represents the mass flowbehavior for a setting with low valve lift. The second, steeper linearslope, which is designated as “Std”, represents the mass flow behaviorfor a setting with higher valve lift.

According to the prior art, the higher setting for the valve lift isalways used whenever a large air mass needs to be fed to the internalcombustion engine in the upper load range.

Based on the maximum rotational speed, a power limiting of the internalcombustion engine takes place due to the fact that the pressure in theintake manifold is reduced by means of a throttle valve. As a result ofthe lower intake manifold pressure, given the same opening lift for theinlet valve a smaller air mass enters the cylinder. In order to maintainthe ideal, stoichiometric fuel/air ratio the injected fuel quantity isreduced together with the air mass taken in.

In the case of a further reduction in rotational speed and torque, thechangeover to a smaller valve lift takes place at a predefinablethreshold n₃ stored in the engine control unit. In order to keep the airmass taken in constant, the throttle valve is simultaneously openedagain. This opening causes a pressure rise in the intake manifold. As aresult, the throttling losses are reduced. Furthermore, the inflow speedof the air taken in at the inlet valve is increased. The turbulenceproduced as a result improves the intermixing of the fuel/air mixtureand thus the combustion.

Based on the switching rotational speed n₃, the rotational speed and thetorque can now be further reduced by successively closing the throttlevalve and reducing the injected fuel quantity correspondingly. In thismanner the internal combustion engine can be throttled right down to theidling rotational speed.

FIG. 2 shows emphasized in bold print the particular valve lift which isset on account of the selection criterion according to variousembodiments for the intake manifold pressure. On the basis of thehighest power and the highest rotational speed of the internalcombustion engine, the greatest possible available valve lift must inturn be chosen. Up to the first switching point n₃ the power and thetorque of the internal combustion engine are reduced by restricting theair supply.

At switching point n3 an operating state is reached which can berealized both with a large valve lift and also with a small valve lift.Therefore, the operating states between the switching points n₁ and n₃can be utilized for the torque-neutral changeover of the valve lift. Forselection of the valve lift to be used in each case, the criterionwhereby the internal combustion engine should be operated as dethrottledas possible is applied. An opened throttle valve expresses itself in ahigher intake manifold pressure than arises when the throttle valve isclosed.

The engine control unit therefore switches dynamically between the twovalve lift settings in the range between the switching points n₁ and n₃,such that the greater intake manifold pressure results in each case. Asa result, between the switching points n₁ and n₃ the internal combustionengine is always situated in the optimum efficiency range.

It is proposed in particular according to various embodiments to alsochange over to the greater valve lift on a case by case basis betweenthe switching points n₁ and n₂, at which according to the prior art achangeover to the greater valve lift never took place. If the internalcombustion engine is regarded as a vacuum pump which evacuates thevolume of the intake manifold, then the throughput of this pump dropsoff as the rotational speed decreases. It has been found, completelysurprisingly, that in the case of an operating point below theintersection point n₂ of the mass flow characteristics the pumping speeddrops off strongly and as a result of this the intake manifold pressure,which falls as restriction increases, rises again. It is thus alsopossible to operate the engine efficiently with a large valve lift anddethrottling in the interval between the switching points n₁ and n₂.Only at rotational speeds which lie beneath the threshold n₁ is atorque-neutral changeover from large valve lift to small valve lift nolonger possible. Therefore, as a precaution, at switching threshold n₁ achangeover to small valve lift is effected and the engine has its torquedelivery limited by closing the throttle valve.

As a result of this premature changeover at the threshold n₁ a situationis avoided whereby a driver who at the operating point n_(B) chosen byway of example is running his internal combustion enginefuel-efficiently with a large valve lift, in the case of a further,considerable lowering of the rotational speed gets into an operatingstate which is not feasible with a large valve lift. On the basis ofn_(B), a changeover to a small valve lift would only be possible if therotational speed were to rise again beforehand up to the threshold n₁.Such a rise in rotational speed and torque conflicting with the driverinput would however be surprising and thus dangerous. At the rotationalspeed n_(B) and a large valve lift the operating state thereforeconstitutes a blind alley which will not be actuated by the controlunit. In the case of a reduction in rotational speed below n₁, thesmaller valve lift is always chosen. In analogous fashion, an operatingpoint above n₃ is only actuated with the large valve lift.

The invention has been described above with reference to functionalunits. The person skilled in the art will implement these functionalunits from case to case as hardware modules or in the form of software.For example, the method according to the invention can be implemented bymeans of a microprocessor, a microcontroller, an ASIC or an FPGA.

1. A method for controlling an internal combustion engine with variablevalve lift and including at least two valve lifts, comprising thefollowing steps: determining the pressure in the intake manifold for afirst, current valve lift; determining the pressure in the intakemanifold corresponding to a changeover of the valve lift from thecurrent valve lift to a second valve lift; comparing the two pressuresto determine which of the first and second valve lifts provides a higherintake manifold pressure; determining whether a valve lift selectioncriterion for a valve lift to be set is met; and if the valve liftselection criterion is not met, selecting for use the valve liftdetermined to provide the higher intake manifold pressure; and if thevalve lift selection criterion is met, selecting for use the valve liftdetermined to provide the lower intake manifold pressure.
 2. The methodaccording to claim 1, wherein the valve lift determined to provide thelower intake manifold pressure is selected if the current torquedelivery in the valve lift determined to provide the higher intakemanifold pressure cannot be achieved.
 3. The method according to claim1, wherein the valve lift determined to provide the lower intakemanifold pressure is selected if a registered driver input cannot beimplemented in the valve lift determined to provide the higher intakemanifold pressure.
 4. The method according to claim 1, wherein the valvelift determined to provide the lower intake manifold pressure isselected in order to prevent the internal combustion engine fromentering an operating state which cannot be set with the two possiblesettings for the valve lift.
 5. The method according to claim 1, whereinthe internal combustion engine is operated with the valve liftdetermined to provide the higher intake manifold pressure if theinternal combustion engine is operated at an operating point having arotational speed below the intersection point of two mass flow curves.6. The method according to claim 5, wherein at the operating pointhaving a rotational speed below the intersection point of two mass flowcurves approx. 5% to approx. 40% of the maximum torque is delivered. 7.The method according to claim 5, wherein at the operating point having arotational speed below the intersection point of two mass flow curvesapprox. 10% to approx. 30% of the maximum torque is delivered.
 8. Themethod according to claim 1, wherein the current pressure in the intakemanifold and the pressure resulting after a valve lift changeover aredetermined at predefinable time intervals.
 9. The method according toclaim 8, wherein the time interval is approx. 5 ms to approx. 100 ms.10. The method according to claim 1, wherein the current pressure in theintake manifold and the pressure resulting after a valve lift changeoverare determined in an event driven manner.
 11. The method according toclaim 10, wherein the event is selected from at least one of the groupconsisting of: a driver input, a change in the drive position, theswitching state of a consumer load, and the intervention of a slipcontrol facility.
 12. A device for controlling an internal combustionengine with variable valve lift and including at least two valve lifts,comprising; a facility for registering the torque and also the pressurein the intake manifold corresponding to a first valve lift, means forpredetermining the pressure corresponding to a changeover of the valvelift from the first valve lift to a second valve lift, means forcomparing the intake manifold pressure corresponding the first valvelift with the intake manifold pressure corresponding the second valvelift; and means for setting the valve lift for the purpose of selectingthe valve lift at which the higher pressure is reached in the intakemanifold, based on the comparison between the intake manifold pressurescorresponding to the first and second valve lifts, unless means forpredetermining a valve lift nominal value require the use of the valvelift at which the lower pressure is reached in the intake manifold. 13.The device according to claim 12, wherein the means for predeterminingthe valve lift nominal value are set up in order to check whether thecurrent torque delivery can be achieved with the valve lift at which thehigher pressure is reached in the intake manifold.
 14. The deviceaccording to claim 12, wherein means are provided for registering adriver input and the means for predetermining the valve lift nominalvalue are set up in order to check whether a registered driver input canbe realized with the valve lift at which the higher pressure is reachedin the intake manifold.
 15. The device according to claim 12, whereinthe means for predetermining the valve lift nominal value are set up inorder to check whether torque-neutral changeover of the valve lift ispossible.
 16. The device according to claim 12, wherein at least one ofthe means for predetermining the valve lift nominal value and the meansfor predetermining the pressure following a changeover of the valve liftcomprise at least one of a numeric characteristic map matrix, amicroprocessor, and a neural network.
 17. The device according to claim12, wherein a timer is provided for determining a predefinable period oftime.
 18. A motor vehicle comprising a device for controlling aninternal combustion engine with variable valve lift and including atleast two valve lifts, comprising a facility for registering the torqueand also the pressure in the intake manifold corresponding to a firstvalve lift, means for predetermining the pressure corresponding to achangeover of the valve lift from the first valve lift to a second valvelift, means for comparing the intake manifold pressure corresponding thefirst valve lift with the intake manifold pressure corresponding thesecond valve lift; and means for setting the valve lift for the purposeof setting selecting the valve lift at which the higher pressure isreached in the intake manifold, based on the comparison between theintake manifold pressures corresponding to the first and second valvelifts, unless means for predetermining a valve lift nominal valuerequire the use of the valve lift at which the lower pressure is reachedin the intake manifold.
 19. The motor vehicle according to claim 18,wherein the means for predetermining the valve lift nominal value areset up in order to check whether the current torque delivery can beachieved with the valve lift at which the higher pressure is reached inthe intake manifold.
 20. The motor vehicle according to claim 18,wherein means are provided for registering a driver input and the meansfor predetermining the valve lift nominal value are set up in order tocheck whether a registered driver input can be realized with the valvelift at which the higher pressure is reached in the intake manifold.